1. Field of the Invention
The present invention relates to a controller for a welding machine in which workpieces are pinched between a pair of electrodes through which welding current is supplied to the workpieces to cause the workpieces to be heated and welded.
2. Description of the Prior Art
It has been well known that welding current and pinching force must be properly controlled to assure a good welding result in such a type of welding machine.
For example, Japanese Patent Publication No. 62-18273 has proposed a technique in which a target welding resistance curve is predetermined, and welding current is so controlled that resistance may be varied along the target curve. Japanese Laid-Open Patent Publication No. 57-146485 discloses a technique in which resistance is differentiated with time, and the result thus obtained is compared with a predetermined value so as to increase or decrease welding current in accordance with the comparison result. Japanese Laid-Open Patent Publication No. 58-181488 discloses a technique in which interelectrode voltage is so controlled as to be varied along a predetermined target curve.
In the prior art, a target curve, a predetermined value or the like is used to control welding current. In other words, a reference value in any sense is used. Such a reference value must be, of course, changed in correspondence with variations in, for example, material, thickness, number, coating layer or others of the workpieces.
Therefore, proper welding current, pinching force or the like must be experimentally obtained or calculated for each specific application of the welding machine. This causes a substantial burden to a user, manufacture or dealer of such a welding machine. Particularly in these days, where workpieces have a tendency of variously changing in material, thickness, coating layer or others, determination of a reference value for each welding condition has caused a considerable burden.
In order to meet various conditions, reference values corresponding to respective welding conditions must be stored, causing the requirement of a large memory as well as complicating the operation for controlling the machine which may disadvantageously degrade facility of the machine.
Even if the workpieces are uniform, repeated welding operations may cause changes with time such as an increase of the electrode diameter, and the process for correcting such a change must be prepared preliminary to each welding condition.
Japanese Patent Publication No. 62-18273 has proposed a technique in which, in case a preceding welding time is too long, the welding current is increased in the following welding, while, in case the preceding welding time is too short, welding current is decreased in the following welding. This technique can adjust the welding current to an optimum with no need of preliminarily defining reference values, but is effective only when uniform welding operations are repeatingly carried out. For example, in case of welding electrodes attached to a robot or the like to carry out welding at different positions, it is impossible to increase or decrease the current value in accordance with the result of the preceding process as taught in the above described Japanese Patent Publication No. 62-18273.